Method of manufacturing low-carbon chromium alloys



Patented Feb. 6, 1934 ME THOD OF MANUFACTURING LOW- CARBON CHROMIUM ALLOYS Ture Robert Haglund, Stockholm, Sweden No Drawing.

Application September 12, 1932,

Serial No. 632,865, and in Sweden September 11 Claims.

This invention relates to a method of manufacturing low-carbon chromium alloys, such as ferro-chrome and chromium-alloyed iron and steel, in which a chromium alloy having a comparatively high content of carbon is refined by means of a slag bath containing chromium oxide and heated by a supply of electric energy.

The invention consists, chiefly, in this that the temperature of the upper and lower layers of the slag is equalized by a stirring of the slag bath by means of carbon monoxide developed in the reaction between easily reducible oxides, such as FeO and CrzOa, contained in the slag and a material supplied in the form of powder, grains,

cuts or the like and containing the carbide of a metal which shall form an ingredient of the fin-- ished alloy, said material being supplied continuously during a long period or in suitable portions with short intervals for the purpose of securing 2 a rather uniform and effective stirring of the slag bath.

In the manufacture of chromium alloys by refining a high-carbon chromium alloy by means of a slag bath containing chromium oxide great difiiculties were hitherto caused by this that the refining operation has a tendency to take place intermittently whereby the generation of gas often will take place with such violence that the slag as well as the metal are thrown up from 0 the furnace or towards its walls where they solidify. Generally the known method is carried out in the following manner. The high-carbon chromium alloy is first molten below a slag comparatively low in chromium oxide and is then decarbonized by a supply of large quantities of chromium ore. As chromium ore has a rather high specific gravity the additions thereof sink down to the lowermost layer of the slag bath and cause said layer to solidify. The slag which is heated from above by means of electric energy supplied by electrodes will now be superheated in its upper layer and finally the lower slag layer begins again to melt. As soon as connection between the subjacent chromium alloy and the 5 molten portion of the slag bath again occurs, a reaction with a rather strong gas generation sets in whereby the slag is stirred because of the fact that superheated slag suddenly comes into contact with the metal. The reaction then takes a nearly explosion-like course whereby metal and slag are thrown from the furnace. The slag bath is on account thereof highly cooled and the lower layer thereof solidifies again. A quiet period without refining now sets in until the solidified portion of the slag above the metal again has molten when a new explosion-like refining period begins. Said inconveniences are removed by the present invention, inasmuch as in working according to the invention the entire slag bath obtains a comparatively uniform temperature, wherebyvthe solidification of the slag bath is avoided. According to the invention the slag is kept in a completely molten state also in the layer which is in direct contact with the metal, and the refining, therefore, takes place uniformly and effectively without any material disturbances.

The method can be used, as mentioned above, for the manufacture of low-carbon ferrochrome as well as of chromium-alloyed, low-carbon iron and steel. In the manufacture of low-carbon ferro-chrome a ferro-chrome having a. comparatively high content of carbon, as for instance 610% C. is, preferably, used as starting material. The ferro-chrome is crushed in such manner that at least a considerable part of the powder, for instance the half thereof or more, has a grain size less than 0.5 mm. The refining is carried out in an electric furnace to which the current is supplied by means of electrodes provided above a slag bath containing chromium oxide and, preferably, movable up and down. The slag bath used for the process should have a high melting point and further a comparatively large molten bath surface. It has been found possible to comply with these two conditions in a satisfactory manner by using electrodes having a large cross-sectional area and a comparatively low average load. The cross-sectional area of the electrodes should thus not be less than 70 dm. measured on the electrode before use. The kw.-load per dm. should not exceed 10 kw. A good result has been obtained in the operation of a three-phase furnace having electrodes of each 1350 mm. diameter and a load on each electrode of about 1000 kw. In the said manner the risk of local super-heatings of the slag bath is essentially reduced. To the said slag bath the pulverized ferro-chrome is added either continuously or at short intervals in small portions. The quantity added per unit of time is chosen in such manner that the generation of carbon monoxide at the reaction setting in between the chromium oxide of the slag and the carbon of the ferro-chrome imparts to the slag bath an effective but nevertheless comparatively uniform stirring whereby the advantage is attained that the temperature of the slag will be practically uniform throughout. In this manner any intermittent explosion-like progress of the reaction is avoided and the advantage is attained that the slag exerts an'effective refining action not only ,on metal particles suspended therein but also on the metal bath below the slag. Preferably, the ferro-chrome is entirely supplied in a pulverized state. On account of the slag bath being kept in a vigorous movement it is, however, possible without risk for any greater disturbances'to supply a part of the ferro-chrome in the form of pieces. Such additions of pieces should be made in portions during the same period as the additions of pulverized ferro-chrome are made. When the addition of the ferro-chrome is finished the bath is subjected to a continued heating, preferably, until the movement of the slag begins to cease or is considerably reduced, and then the metal is drawn on. The slag is, preferably, allowed to rest in the furnace and to serve as refining slag for a fresh quantity of ferro chrome. Because of the reaction of the carbon of the high-carbon ferro-chrome with the chromium oxide of the slag the content of chromium oxide in the slag has a tendency to be reduced which in its turn tends to give the slag a composition which is less suitable for the refining operation, it has been found necessary to compensate said tendency. Such compensation is, preferably,

eiiected by a supply of material rich in chromium oxide, as for instance chromium ore, chromium oxide or a material containing chromium oxide and obtained by oxidation of ferro-chrome. *It is suitable to use such additions containing chromium oxide which have a comparatively low content of slag forming oxides inasmuch as it then is more easy to secure the desired composition of the slag bath. Adjustment of the chromium oxide content of the slag can also be obtained by oxidation of a part of the ferro-chrome present in the furnace. For this purpose air or oxygen may be blown into the metal bath or provisions may be made for having such" an ample access of air to the surface of the slag bath that a portion of the molten metal particles suspended in the slag bath are oxidized by the infiuence of the air. It is suitable to use raw materials having such a low content of sulphur that, apart from the possible sulphur refining action of the slag; a sulphur content below 0.025% would be obtained in the finished alloy. As a rule some lime is, however, added to the slag in order to facilitate the securing of a low content of sulphur in the finished product. If the content of chromium oxide in the slag is increased in an undesirable manner an adjustment can be made by a supply of a carbonaceous'reducing agent, such as cokes, anthracite or charcoal. If the slag in the furnace is increased above the desired quantity the excess is drawn off.

As already mentioned, iron and steel alloyed with chromium, as for instance the so-called rustless and fireproof types the chromium content of which is lying between about 11 and 30% can be manufactured with advantage according to the invention. Also in this case a ferro-chrome having a high content of carbon, as for instance 6-10%, and a content of chromium above 60% is, preferably, used as high-carbon raw material, though, of course, also other sorts having lower contents of carbon and chromium may be used. The ferro-chrome is supplied in a pulverized state and in a similar manner as in the method of manufacturing low-carbon ferro-chrome above described. The iron required for the process is, preferably, added in metallic form, as for instance in the form of forge-iron scrap, pig-iron or spongelron. If the additions consist of an iron comparatively low in carbon the latter can. preferably, be molten before or after the period during which the additions of powdered material containing metal carbide are made, and it can, if desired, be added in the form of comparatively large pieces. The same applies also for the re melting of scrap from chromium-alloyed iron and steel manufactured according to the process. It has, however, been found that it is suitable to supply at least the chief part of such low-carbon iron or iron alloy in portions during the period in which the material containing metal carbide is supplied as powder, grains, cuts, or the like. If iron is supplied in the form of pig-iron or forgeiron or steel having a higher content of carbon than what is desired in the finished alloy such iron is, preferably, added as grains or powder, as for instance crushed grains, if it consists of pigiron, or as cuts if it consists of forge-iron or steel. Such additions of high carbon iron in the formof grains; powder or cuts may be used together with pulverized ferro-chrome as admixture containing metal carbide for the purpose of effecting a temperature equalization in the slag. Alternatively, they may be used alone for such purpose. If pulverized ferro-chrome as well as iron are used for the equalization of the temperature they may be charged either mixed with each other or during the same period, or the additions of ferrochrome can'be supplied during one period and the additions of finely divided iron during another period. If chromium-nickel-iron-alloys, as for instance of the fireproof or rustless sorts, are to be manufactured according to the invention metallic nickel, can, preferably, be added at the beginning of the melting operation. In order to secure that the chromium content will not be too low the charge in manufacturing chromiumalloyed iron and steel may be chosen in such manner that at first a somewhat too high content of chromium is obtained. After the refining is finished or in connection with the final refining a quantity of iron ore or other material rich in iron oxide sufiicient to :oxidize the excess of chromium is in such case added to the charge. It has been found that such method of performing the final adjustment in respect of the chromium content gives the desired result more rapidly and safely than if the final adjustment takes place by an addition of metallic iron. An increase of the chromium content if this would be required may be obtained by a supply of a further quantity of ferro-chrome or by reducing a part of the chromium oxide content of the slag, for instance by an addition of aluminium, silicon-iron, siliconchromium or other alloys containing silicon or aluminium, or by an addition of carbonaceous reducing means. Also in the manufacture of chromium-alloyed iron and steel it is suitable to use raw materials having such a low content of sulphur that the alloy, apart from the sulphurrefining action of the slag would obtain a content of sulphur less than 0.025%, thus rendering it possible to compose the slag in a manner most suitable for the refining operation, i. e. without paying any greater regard to the requirement that the slag shall have a sulphur refining action. In accordance .with the statements about the manufacture of low-carbon ferro-chrome according to the invention there is also in the manufacture of chromium-alloyed iron and steel 2. tendency of the chromium oxide content of the slag to be reduced by the influence of the carbon content of the added high-carbon ferro-chrome and by the high-carbon iron possibly added. Also in this case it is suitable to counteract said tendency by providing for such ample access of air to the molten surface of the slag bath around the electrodes that a part of the metal particles suspended in the slag bath are oxidized by the influence of the air. An increase of the content of chromium oxide in the slag bath may, however, also partly or wholly be obtained in another manner, as for instance by a supply of iron ore, chromium ore, or other material rich in chromium.

By keeping the content of silica in the slag low in comparison with its content of chromium oxide in the process it is possible to manufacture chromium-alloyed iron having a content of chromium lying for instance between 10 and 25%, a content of carbon less than 0.1% and a content of silicon between 0.1% and traces. It is suitable to keep in such manner the silicon content below 0.15% as it has been found that in such case a chromium-alloyed iron having very good properties as regards the tensile strength as well as the possibility of working the material is obtained.

The composition and temperature of the slag have a great importance for the process and the slag should, preferably, be kept easily fluid during the whole melting operation. In order to secure a rather rapid decarbonization by reaction between chromium oxide in the slag and carbide in the metal it is required that the slag is heated to a high temperature. The higher the temperature of the slag is, the more rapidly and completely will the reaction take place. Within certain limits an increase of the temperature, therefore, causes a reduced consumption of electric energy per unit of product manufactured. At quite too high temperature, however, gasifying phenomena having a disturbing influence on the process, occur. It has thus been found that the temperature of the slag bath during the refining operation should not be less than 1700 C. and it should, preferably, be heated to a temperature between 1800 and 2100 C.

Another important factor is the content of 'chromium oxide in the slag. The content of chromium oxide should thus normally not be less than 15% and is, preferably, kept between the limits 25 and 60%. It has further been found suitable to adjust the composition of the slag in other respects in such manner that its content of chromium oxide is not by the presence of high percentages of strongly basic oxides bound to the latter with such strength that the chromium oxide will be less reactive. Counted in molecules the total content of oxides of calcium and magnesium in the slag should, therefore, be at most 1.5 times the total content of oxides of chromium, aluminium and silicon in the slag and the content of the first-mentioned oxides is, preferably, so chosen that it does not amount to the content of the latter oxides. With regard to the desirability of obtaining a low content of silicon in the alloy the content of silica in the slag is generally kept low, or less than 20% and, preferably, within the limits 53-15%. When the content of chromium oxide is high, for instance greater than 30%, it is under some circumstances possible to use with advantage also slags having a considerably higher content of silica than that now mentioned.

In order to facilitate the refining operation it is suitable to keep the partial pressure of'the carbon monoxide low at least above a part of the molten surface of the slag bath. This can be attained in a very simple manner by combusting the carbon monoxide immediately above a part of the surface of the slag bath, for instance by means of air. The furnace is, preferably, arranged in such manner that the air required for such purpose is admitted by natural draft. The composition of the slag can be adjusted by suitable additidns containing the oxides in respect of which it is desirable to enrich the slag, as for instance chromium ore or other material rich in chromium, lime, quartz, chamotte, bauxite, alundum, etc. An adjustment of the composition of the slag may, however, also be attained, as above described, by oxidation of chromium in the metal bath or of metal particles containing chromium suspended in the slag, or by reduction of chromium oxide by means of additional reducing carbon.

In drawing oif metal the slag is as a rule left in the furnace to serve as refining slag for a fresh charge. According as the volume of the slag bath is increased above the desired value the excess is drawn oif, preferably by a separate slag tapping. If raw materials low in sulphur are used, it is possible to work for a long time with the same slag bath without regeneration thereof. The required regeneration is, preferably, carried out in such manner that provision is made for a slow increase of the slag bath and that the excess is drawn off from time to time. In the manufacture of low-carbon ferro-chrome chro mium ore is, generally, added to increase the chromium content of the slag. As the chromium ore contains considerable percentages of oxides of magnesium and aluminium a slow increase of the slag bath is obtained in this manner. In the manufacture of chromium-alloyed iron and steel the adjustment of the chromium content in the slag is, preferably, effected by additions of rich iron ore, so that in such case no essential increase of the volume of the slag bath needs occur unless this is particularly desired, for instance on account of a desire to reduce the sulphur content.

Special alloying substances, such as nickel may occasionally be added in molten or unmolten state either in the refining furnace proper or wholly or partly in a ladle, mixer or a separate furnace in which the alloy is tapped for adjustment. When the manufacture of an alloy having a rather high content of nickel is aimed at and nickel is added partly as scrap of nickel-alloyed iron, partly as metallic nickel or other alloy high in nickel the process is, preferably, carried out in such manner that the scrap containing nickel is supplied to the refining furnace while the additions of metallic nickel are made in the ladle, or in the mixer, or in the adjusting furnace. The object of such subdivision of the additions containing nickel is in the first line to facilitate when necessary a change of the manufacturing process to a product low in nickel or free from nickel. As the refining furnace ought not be tapped wholly empty of metal a rapid change to a product low in Ni would be rendered diflicult by a high content of nickel in the metal present in the refining furnace. If the scrap containing nickel has a comparatively high content of carbon it should be supplied 'in unmolten state and the additions thereof be made in small portions 1 during a long period of time. On the other hand, it is possible to add scrap low in carbon as well as other low-carbon iron forming part of the charge in molten or unmolten state. In

drawing of! the alloyed iron or steel from the refining furnace it has such a high temperature that it generally is possible to dissolve even considerable quantities or additions, for instance of metallic nickel, in the ladle, without a need of beforehand melting such additions.

If smaller or greater additions of manganese should be made such additional ingredients containing manganese should not be supplied until after the tapping from the refining furnace as the manganese would volatilize to a great part at the high temperature prevailing in said furnace. Minor additions of silicon may also be supplied, preferably, after the tapping, for instance in the ladle.

Example.To a refining slag bath containing 40% CrzOa and 14% SlOz electric energy was supplied by means of three electrodes mounted movably up and down above the bath, each having a diameter of 1350 mm. and each being loaded with 900 kw. To said slag bath a mixture of 2500 kgs. of iron cuts and 1200 kgs. of pulverized ferro-chrome containing 60% Cr and 7% C were added in small portions during a period of 2 hours. During a period of further 20 minutes 500 kgs. of sponge-iron in pieces were added in portions. The heating was continued for a further period of 20 minutes. A metal sample taken up from the furnace then proved a content of 15.4% Cr and less than 0.1% of each of Si and C.

What I claim is:-

1. A process of decarburizing chromium alloys by reaction with a slag bath containing chromium oxide, which comprises heating said slag bath by means of electrodes operating in electrical connection with the top portion of said slag bath, and successively adding fine particles of a chromium alloy high in carbon, said added particles being spread in the slag by means of the stirring effect caused by the carbon monoxide evolved in the slag by the reaction between the chromium oxide of the slag and the carbon content'of the added particles of chromium alloy.

2. In a process as claimed in claim 1, the step which comprises supplying to the slag bath a part of the chromium alloy to be decarburized as comparatively large pieces.

3. In a process as claimed in claim 1, the step which comprises subjecting the slag bath to a continued heating after the adding of high-carbon chromium alloy is finished.

4. A process of refining high-carbon ferrochromium, comprising heating a slag bath containing chromium oxide by means of electrodes operating in electrical connection with the top portion of said slag bath to a temperature exceeding 1700 C., and adding succemively pulverized ferrochromium, with a carbon content of 6% or more, to said slag bath.

5. A process of decarburizing chromium alloys by reaction with a slag bath containing chromium oxide, comprising heating said slag bath by means of electrodes operating in electrical connection with the top portion of the bath, regulating the composition of the slag in such manner that it contains a maximum of 15% of silica, from 25 to 60% of chromium oxide, and the remainder comprising mainly oxides of aluminium, magnesium and calcium, and successively spreading pulverized ferrochromium high in carbon in the fluid slag mass.

6. A process of decarburizing chromium alloys by reaction with a slag bath containing chrcmium oxide, comprising heating said slag bath by means of electrodes operating in electrical connection with the top portion of the bath, reg ulating the composition of the slag bath in such. manner that it contains a maximum of 15% of silica, from 25 to 60% chromium oxide, and the remainder comprising mainly oxides of aluminium, magnesium and calcium, the total content of oxides of calcium and magnesium being less than 1.5 times the total content of oxides of chromium, aluminium and silicon, successively adding pulverized high-carbon ferrochromium, and continuing the heating of the slag bath after the adding of high-carbon chromium alloy is finished.

7. A process of producing chromium alloyed iron and steel, comprising heating a slag bath containing chromium oxide by means of electrodes operating in electrical connection with the top portion of said slag bath, adding a highcarbon chromium alloy and iron, and successively spreading small particles of high-carbon alloy in the fluid slag mass by means of the stirring effect of carbon monoxide developed by the reaction between the slag bath and the high-carbon alloy.

8. A process of producing chromium alloyed iron and steel, comprising heating a slag bath containing chromium oxide by means of electrodes operating in electrical connection with the top portion of the slag bath to a temperature exceeding 1700" C., successively adding pig iron and pulverized high-carbon ferrochromium, and decarburizing the alloy by means of the chromium oxide of the slag.

9. A process of producing low-carbon chromium alloyed iron and steel, in which the chromium added is at least partially in the form of an alloy containing a high percentage of chromium carbide, comprising successively adding the latter alloy at least a portion of which is in pulverized form to a slag bath consisting mainly of oxides of chromium, aluminium, magnesium, calcium and silicon, the content of chromium oxide being maintained during the whole operation at 25 to 60% and the content of silica being maintained below a maximum of 15%, and maintaining the slag bath during the operation at a temperature exceeding 1700 C. by means of electrodes operating in electrical connection with the top portion of the slag bath.

10. In the manufacture of low-carbon chromium alloys as set forth in claim 1, the step which comprises admitting air to the surface of the slag bath to such extent that the additions of highcarbon chromium alloy in form of small particles are oxidized before being dissolved in the metal bath below the slag.

11. In a process of producing low-carbon chromium alloyed iron and steel, in which the chromium is added at least partially in the form of a high-carbon chromium alloy, by means of a chromium oxide slag heated by means of electrodes operating in electrical connection with the top portion of the slag bath, the steps of successively adding one part of the charge in the form of small particles of a high-carbon alloy and another part of the charge in the form of a lowcarbon alloy, at least the larger portion of the low-carbon material being added before the charging of the high-carbon material is finished.

ROBERT HAGLUND.

Ill 

